Gas diffusion layer for fuel cell, comprising spun carbon nanofiber layer

ABSTRACT

The present invention relates to a gas diffusion layer including a carbon nanofiber spun layer for a fuel cell, a membrane-electrode assembly including the gas diffusion layer for a fuel cell, and a fuel cell including the membrane-electrode assembly.

TECHNICAL FIELD

The present invention relates to a gas diffusion layer including a carbon nanofiber spun layer for a fuel cell, a membrane-electrode assembly including the gas diffusion layer, and a fuel cell including the membrane-electrode assembly.

This application claims the benefit of Korean Patent Application No. 10-2017-0080431 filed on Jun. 26, 2017, and the entire contents of which are incorporated herein by reference.

BACKGROUND ART

A fuel cell is an electrochemical cell that converts chemical energy produced by oxidation of fuel into electrical energy. Recently, various investigations have focused on the development of fuel cells as well as solar cells and the like in order to overcome problems such as consumption of fossil fuels, the greenhouse effect and global warming caused by carbon dioxide, and the like.

A fuel cell generally converts chemical energy into electrical energy through oxidation and reduction of hydrogen and oxygen. In the fuel cell, hydrogen is oxidized into hydrogen ions and electrons at an anode, and the hydrogen ions diffuse to a cathode through an electrolyte. The electrons travel to the cathode through a circuit. At the cathode, water is produced through reduction of the hydrogen ions, electrons, and oxygen.

The gas diffusion layer of the fuel cell serves to introduce a reactive gas (hydrogen, oxygen, etc.) flowing through a separator interposed between the unit cells of the fuel cell or the outside, into a catalytic layer having an electrochemical reaction, and to discharge the condensed water generated by the electrochemical reaction.

If the reaction gas cannot smoothly pass through the gas diffusion layer of the fuel cell, the reaction concentration is lowered to reduce the generation voltage.

Further, if the gas diffusion layer of the fuel cell cannot smoothly discharge the condensed water generated due to the electrochemical reaction, the reaction concentration is also lowered to reduce the generation voltage.

Meanwhile, if the gas diffusion layer of the fuel cell does not smoothly discharge the generated heat, the electrolyte-passing electrolyte membrane is dried to lower the ion conductivity, thereby increasing the resistance loss to reduce the generation voltage.

Therefore, the gas diffusion layer of the conventional fuel cell has problems in that the electrolyte membrane is dried and the heat generated by the electrochemical reaction is not smoothly discharged when the gas diffusion layer has the increased porosity for promoting the inflow of the reaction gas and promoting the discharge of the condensed water. These problems result in a limitation that the gas diffusion layer cannot have a high porosity above a certain level and that the electric conductivity is low so that the power generation efficiency cannot be increased.

Further, in the gas diffusion layer of the conventional fuel cell, the fine pores of the polymer membrane are controlled to a fine size by coating the fine carbon particles in order to maintain the wet state of the electrolyte membrane. However, there is a problem in that the pores of the electrolyte membrane are unevenly distributed so that the reaction gas cannot flow smoothly to reduce the power generation efficiency, and there is a difficulty in thickness control due to the multilayer laminated structure.

DISCLOSURE Technical Problem

The present invention is to provide a gas diffusion layer for a fuel cell which has a uniform pore distribution and a high porosity while exhibiting excellent heat transfer and electric conduction efficiency to have excellent power generation efficiency, a membrane-electrode assembly including the gas diffusion layer and a fuel cell including the membrane-electrode assembly.

Technical Solution

The present invention provides a gas diffusion layer including a carbon nanofiber spun layer for a fuel cell, in which the carbon nanofiber spun layer is formed by electrospinning a polymer composition.

Further, the present invention provides a membrane-electrode assembly for a fuel cell, the assembly including: an electrolyte membrane; and an anode electrode and a cathode electrode facing each other with the electrolyte membrane interposed therebetween, in which each of the anode electrode and the cathode electrode includes the gas diffusion layer and a catalyst layer.

Further, the present invention provides a fuel cell including: a stack including one or more of the membrane-electrode assemblies and a separator interposed between the membrane-electrode assemblies; a fuel supply unit for supplying fuel to the stack; and an oxidant supply unit for supplying an oxidant to an electricity generating unit.

Advantageous Effects

Since the gas diffusion layer according to the present invention has a high porosity and a uniform pore distribution, it promotes the inflow of the reaction gas and the discharge of the condensed water and also has excellent heat transfer and electric conduction efficiency to prevent the electrolyte membrane from being dried. Thus, the membrane-electrode assembly and fuel cell including the same show excellent power generation efficiency.

DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph of the microstructure of the gas diffusion layer produced according to Example 1-2 in Experimental Example 1, taken with a field emission scanning electron microscope at a magnification of 10 k.

FIG. 2 is a photograph of the microstructure of the gas diffusion layer produced according to Comparative Example 1-2 in Experimental Example 1, taken with a field emission scanning electron microscope at a magnification of 10 k.

FIG. 3 illustrates a current-voltage curve obtained by measuring the current-voltage values of each unit cell according to Examples 1-2 to 4-2 in Experimental Example 2.

MODE FOR INVENTION

Hereinafter, the gas diffusion layer for a fuel cell according to the present invention is described.

The gas diffusion layer for a fuel cell according to the present invention includes a carbon nanofiber spun layer.

The carbon nanofiber spun layer according to the present invention is formed by electrospinning a polymer composition.

In one embodiment of the present invention, the electrospinning is performed by applying a voltage of 30 kV to 70 kV, more preferably a voltage of 40 kV to 60 kV, during the spinning of the polymer composition. If the voltage is less than 30 kV, the splitting of the fibers is not actively performed, and the volatility of the solvent is lowered. If the voltage exceeds 70 kV, a tip-trouble occurs at the tip of the nozzle through which the polymer composition is spun.

In one embodiment of the present invention, the electrospinning is performed at a temperature of 50° C. to 80° C. If the temperature at which the electrospinning is performed is less than 50° C., the viscosity of the polymer solution becomes high to prevent the polymer from being spun smoothly. Therefore, it may not ensure mass productivity. If the temperature at which the electrospinning is performed is more than 80° C., the solvent is volatilized in the polymer solution so that the composition of the polymer solution may be changed. Further, the pressure in the solution tank due to solvent volatilization may increase, resulting in risk of explosion.

In one embodiment of the present invention, the fiber of the electro-spun carbon nanofiber spun layer has the average diameter of 0.01 μm to 2 μm, more preferably 0.02 μm to 1 μm. If the average diameter of the fiber is less than 0.01 μm, the size of the gap between the fibers decreases to reduce the gas permeability. If the average diameter of the fiber exceeds 2 μm, the size of the gap between the fibers increases so that the foreign substances present in the gas pass through the gap to accumulate in the cell stack, resulting in a decrease of the performance as the gas diffusion layer of the fuel cell.

In one embodiment of the present invention, the electrospinning is performed by applying pressure to the container while a voltage is applied between a tip which is an opening of the container storing the polymer composition and a current collecting plate spaced apart from the tip in the gravity direction.

In one embodiment of the present invention, the spacing distance between the tip and the current collecting plate is 10 cm to 20 cm, preferably 12 cm to 16 cm. If the spacing distance is less than 10 cm, the spinning is terminated before splitting of the fibers so that the residual solvent is left, and the nanofibers are melted due to the residual solvent, resulting in desired carbon nanofiber deformation. When the spacing distance exceeds 20 cm, the magnetic field formation between the current collecting plates becomes unstable so that the carbon nanofiber layer is not formed.

In one embodiment of the present invention, the polymer composition includes at least one selected from the group consisting of a polyacrylic resin such as polymethyl methacrylate (PMMA), polystyrene (PS), polyacrylic acid (PAA) and polyacrylonitrile (PAN); a polyvinyl resin such as polyvinyl chloride (PVC), polyvinyl alcohol (PVA) and polyvinyl acetate (PVAc); a polyester resin such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polybutyleneterephthalate (PBT); Nylon; polycarbonate, polyethylene oxide (PEO); polyurethane (PU), polyvinylidene fluoride (PVdF); poly(vinylidene fluoride)-co-(hexafluoroprop ylene) [P(VDF-HFP)]; poly(vinylidene fluoride)-co-(chlorotrifluoroethylene) [P(VDF-CTFE)], polytetrafluoro ethylene-co-hexafluoro propylene-co-vinylidene fluoride (THV); polyether ether ketone, poly phenylene oxide (PPO); poly phenylene sulfone (PPS); polysulfone (PS); poly ether sulfone (PES); polyimide (PI); polyether imide (PEI); polyamide imide (PAI); polybenzimidazole (PBI); polybenzoxazole (PBO); and polyaramid.

In one embodiment of the present invention, the polymer composition includes polyacrylonitrile, polyvinylidene fluoride, or a combination thereof.

In one embodiment of the present invention, the carbon nanofiber spun layer has a thickness of 20 μm to 200 μm, more preferably 50 μm to 150 μm. When the thickness of the carbon nanofiber spun layer is less than 20 μm, the physical properties of the carbon nanofiber spun layer deteriorate during the heat treatment. When the thickness of the carbon nanofiber spun layer is more than 200 μm, there is a space limitation in the stacking of the gas separation layer on the membrane-electrode assembly after the heat treatment so that an effective catalyst layer necessary for manifesting the performance of the fuel cell may not be formed.

In one embodiment of the present invention, the carbon nanofiber spun layer is formed by heat-treating the carbon nanofiber spun layer formed by the electrospinning.

In one embodiment of the present invention, the heat treatment step is a method including an oxidation stabilization step, a carbonization step and a graphitization step.

In one embodiment of the present invention, the oxidation stabilization step includes the step of introducing the nanofiber spun layer into a first heater having a first temperature range to allow a cyclization reaction of the nanofiber film. The double bonds produced in the oxidation stabilization step reduce the chain breakage and improve the heat resistance in the carbonization step.

In one embodiment of the present invention, the first temperature range is a range that sequentially rises from room temperature to 300±30° C.

In one embodiment of the present invention, the carbonization step includes the step of introducing the nanofiber spun layer into a second heater having a second temperature range of the carbonization, thereby carbonizing the nanofiber film to convert it into a carbonaceous film.

The first temperature range of the carbonization is a range that sequentially rises from 300±30° C. to 1,000° C.

In one embodiment of the present invention, the graphitization step includes the step of introducing the carbonaceous film into a third heater having a third temperature range, the temperature of which is linearly increased, thereby converting the carbonaceous film into a porous graphite film.

The third temperature range is a range that sequentially rises from 1,000° C. to 2,800° C.

In one embodiment of the present invention, the third temperature range includes the temperature range 3-1 of 1,000° C. to 1,500° C., the temperature range 3-2 of 1,500° C. to 2,200° C., and the temperature range 3-3 of 2,200° C. to 2,800° C.

In one embodiment of the present invention, the graphitization step includes the step of moving the carbonaceous film in the transverse direction at the speed of 0.33 mm/seconds to 1.33 mm/seconds in the temperature range 3-1 and heat-treating the carbonaceous film for 1 hour to 4 hours while raising the inside temperature of the third heater at 1° C. to 5° C. per minute.

In one embodiment of the present invention, the organic solvent is acetone; a sulfoxide-based solvent such as dimethylsulfoxide and diethylsulfoxide, a formamide-based solvent such as N,N-dimethylformamide and N,N-diethylformamide, an acetamide-based solvent such as N,N-dimethylacetamide and N,N-diethylacetamide, a pyrrolidone-based solvent such as N-methyl-2-pyrrolidone and N-vinyl-2-pyrrolidone, a phenol-based solvent such as phenol, o-, m-, or p-cresol, xylenol, halogenated phenol, and catechol, an aprotic polar solvent such as hexamethylphosphoramide and γ-butyrolactone, or a mixture thereof.

In one embodiment of the present invention, the organic solvent further includes an aromatic hydrocarbon such as xylene, toluene and methylethylketone.

In one embodiment of the present invention, the organic solvent is diethylacetamide, diethylformamide, acetone, methylethylketone or a mixture thereof.

Hereinafter, a method for producing a gas diffusion layer for a fuel cell according to the present invention is described. Unless otherwise specified, the description of the gas diffusion layer for a fuel cell as described above can be applied to the following method for producing a gas diffusion layer for a fuel cell.

The method for producing a gas diffusion layer for a fuel cell according to the present invention includes the step of electrospinning a polymer composition and forming a nanofiber spun layer.

The step of forming the nanofiber spun layer includes the step of applying a voltage of 30 kV to 70 kV, more preferably of 40 kV to 60 kV, during spinning of the polymer composition.

In one embodiment of the present invention, the step of electrospinning includes the step of applying pressure to the container while a voltage is applied between a tip which is an opening of the container storing the polymer composition and a current collecting plate spaced apart from the tip in the gravity direction.

The method for producing a gas diffusion layer for a fuel cell according to the present invention includes a step of heat-treating the carbon nanofiber spun layer formed by the electrospinning.

In one embodiment of the present invention, the step of heat-treating is a method including an oxidation stabilization step, a carbonization step and a graphitization step.

Hereinafter, the membrane-electrode assembly for a fuel cell according to the present invention is described.

The membrane-electrode assembly for a fuel cell according to the present invention includes an electrolyte membrane for the fuel cell; and an anode electrode and a cathode electrode positioned opposite to each other with the electrolyte membrane interposed therebetween.

In one embodiment of the present invention, the electrolyte membrane may be formed of a perfluorosulfonic acid polymer, a hydrocarbon-based polymer, polyimide, polyvinylidene fluoride, polyethersulfone, polyphenylene sulfide, polyphenylene oxide, polyphosphazene, polyethylene naphthalate, polyester, doped polybenzimidazole, polyether ketone, polysulfone, an acid thereof or a base thereof.

Each of the anode electrode and the cathode electrode according to the present invention includes a gas diffusion layer and a catalyst layer.

In one embodiment of the present invention, the catalyst layer of the anode electrode may include at least one catalyst selected from the group consisting of platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy and platinum-transition metal alloy.

In one embodiment of the present invention, the catalyst layer of the cathode electrode includes platinum.

In one embodiment of the present invention, the catalyst of the anode electrode or the cathode electrode is supported on the carbon-based carrier.

Hereinafter, the fuel cell according to the present invention is described.

The fuel cell according to the present invention includes: a stack including the membrane-electrode assembly and a separator interposed between the membrane-electrode assemblies; a fuel supply unit for supplying fuel to the stack; and an oxidant supply unit for supplying an oxidant to an electricity generating unit.

The separator according to the present invention serves to prevent the membrane-electrode assemblies from being electrically connected, to transfer the fuel and oxidant supplied from the outside to the membrane-electrode assembly, and serves as a conductor for connecting between the anode electrode and the cathode electrode in series.

The fuel supply unit according to the present invention serves to supply fuel to the stack and may include a fuel tank for storing the fuel and a pump for supplying the fuel stored in the fuel tank to the stack.

In one embodiment of the present invention, the fuel is hydrogen or hydrocarbon fuel in a gaseous or liquid state.

In one embodiment of the present invention, the hydrocarbon fuel is methanol, ethanol, propanol, butanol or natural gas.

The oxidant supply unit according to the present invention serves to supply the oxidant to the stack.

In one embodiment of the present invention, the oxidant is oxygen or air.

In one embodiment of the present invention, the oxidant is injected with a pump.

In one embodiment of the present invention, the fuel cell is a polymer electrolyte-type fuel cell or a direct methanol-type fuel cell.

Hereinafter, the present invention is described in more detail by way of Examples. However, these Examples are intended to illustrate the present invention only, and the scope of the present invention is not limited by these Examples.

<Example 1-1> Production of Carbon Nanofiber Spun Layer

900 g of dimethylacetamide (DMAc) was added to 100 g of polyacrylonitrile (PAN) and dissolved to produce a polymer spinning solution (the solution's concentration: 10% by weight).

Thereafter, 6 ml of the produced polymer spinning solution was input into the polymer composition supply container of the electrospinning device manufactured by Oseong Technology Co. from Korea. Then, the distance between the tip, which is the opening of the supply container, and the collecting plate spaced apart from the tip in the gravity direction, was set to 15 cm, a voltage of 30 kV was applied between the tip and the current collecting plate while the temperature in the supply container was controlled to be 70° C. as a constant temperature, and the polymer spinning solution of the supply container was sprayed with pressure for 8 hours, thereby obtaining the carbon nanofiber spun layer having a width of 25 cm, a length of 45 cm and a thickness of 100 μm.

<Example 2-1> Production of Carbon Nanofiber Spun Layer

A gas diffusion layer was produced in the same manner as in Example 1-1 except that the temperature in the supply container for the polymer spinning solution of the electrospinning device was controlled to be 45° C. as constant temperature.

<Example 3-1> Production of Carbon Nanofiber Spun Layer

A gas diffusion layer was produced in the same manner as in Example 1-1 except that the temperature in the supply container for the polymer spinning solution of the electrospinning device was controlled to be 35° C. as a constant temperature.

<Production Example 4-1> Production of Carbon Nanofiber Spun Layer

A gas diffusion layer was produced in the same manner as in Example 1-1 except that the temperature in the supply container for the polymer spinning solution of the electrospinning device was controlled to be 25° C. as constant temperature.

<Example 1-2> Production of Gas Diffusion Layer

The carbon nanofiber spun layer produced in Example 1-1 was heat-treated to produce a gas diffusion layer. The nanofiber spun layer produced in Example 1-1 was introduced into the first heater having the first temperature range to produce a nanofiber film stabilized by the cyclization reaction of the nanofiber film. The first temperature range is a period that rises sequentially up to 300±30° C.

The stabilized nanofiber film was introduced into the second heater having the second temperature range to carbonize the nanofiber film, thereby producing a carbonaceous film. The second temperature range is a period that sequentially rises from 300±30° C. to 1,000° C.

The carbonaceous film produced by the above method was introduced into the third heater having the third temperature range to produce a porous gas diffusion layer. The third temperature range is a period that sequentially rises from 1,000° C. to 2,800° C.

<Example 2-2> Production of Gas Diffusion Layer

A gas diffusion layer was produced in the same manner as in Example 1-2, except that the carbon nanofiber spun layer produced in Example 2-1 was used as the carbon nanofiber spun layer.

<Example 3-2> Production of Gas Diffusion Layer

A gas diffusion layer was produced in the same manner as in Example 1-2, except that the carbon nanofiber spun layer produced in Example 3-1 was used as the carbon nanofiber spun layer.

<Example 4-2> Production of Gas Diffusion Layer

A gas diffusion layer was produced in the same manner as in Example 1-2, except that the carbon nanofiber spun layer produced in Example 4-1 was used as the carbon nanofiber spun layer.

<Example 1-3> Production of Electrolyte Membrane in which Catalyst Layer is Formed

The electrolyte membrane was a Nafion 112 membrane manufactured by DuPont, which is a perfluorosulfonic acid polymer. In order to produce the catalyst ink, a platinum-supported carbon catalyst (Pt/C) was used as the anode and the cathode catalysts. Nafion solution, isopropyl alcohol and water were mixed, and they were mixed with the above catalyst so as to make a catalyst:Nafion dry weight:solvent=1:0.3:20, followed by stirring so as to be well dispersed. Then, the result was uniformly mixed by a high-speed mixer (for 2 hours) to produce a catalyst ink.

The produced catalyst ink was sprayed on one side of the polymer electrolyte membrane using a spray coater to form a catalyst layer having a density of 0.4 mg/cm².

<Example 1-4> Production of Unit Cell

The gas diffusion layers produced in Example 1-2 were superimposed on both sides of the electrolyte membrane produced in Example 1-3. In order to maintain gas sealing property around the membrane-electrode assembly, a 210 μm gasket adhered to the polymer electrolyte portion excluding the electrode portion, and the anode plate having a flow path for supplying hydrogen and uniform pressure and the cathode plate for supplying air and uniform pressure to the membrane-electrode assembly were adhered to the membrane-electrode assembly, thereby producing the unit cell.

<Example 2-4> Production of Unit Cell

A unit cell was produced in the same manner as in Example 1-4, except that the gas diffusion layer produced in Example 2-2 was used as a gas diffusion layer.

<Example 3-4> Production of Unit Cell

A unit cell was produced in the same manner as in Example 1-4, except that the gas diffusion layer produced in Example 3-2 was used as a gas diffusion layer.

<Example 4-4> Production of Unit Cell

A unit cell was produced in the same manner as in Example 1-4, except that the gas diffusion layer produced in Example 4-2 was used as a gas diffusion layer.

<Comparative Example 1-1> Production of Carbon Nanofiber Spun Layer

A carbon nanofiber spun layer with a width of 25 cm, a length of 45 cm and a thickness of 100 μm was produced in the same manner as in Example 1-1, except that the temperature of the polymer composition supply container was not controlled but maintained at 25° C., which was room temperature, during the electrospinning.

<Comparative Example 1-2> Production of Gas Diffusion Layer

A gas diffusion layer was produced in the same manner as in Example 1-2, except that the carbon nanofiber spun layer produced according to Comparative Example 1-1 was used.

<Comparative Example 1-3> Production of Electrolyte Membrane in which Catalyst Layer is Formed

An electrolyte membrane was produced in the same manner as in Example 1-3.

<Comparative Example 1-4> Production of Unit Cell

A unit cell was produced in the same manner as in Example 1-4, except that the gas diffusion layer made using the carbon nanofiber spun layer produced according to Comparative Example 1-1 was used.

<Experimental Example 1> Observation of Microstructure Via FE-SEM

The microstructures of the gas diffusion layers produced according to the methods of Examples 1-2 and Comparative Example 1-2 were photographed using a field emission scanning electron microscope (FE-SEM) with a trade name of SU-70, manufactured by Hitachi, Ltd. The photographs are illustrated in FIGS. 1 and 2, respectively.

The results of observing the microstructure of the nanofibers according to the present invention indicate that the diameters of the fibers were different depending on whether the temperature of the polymer spinning solution was controlled in a certain range during the production of the carbon nanofiber spun layer. As a result, it can be confirmed that when a certain temperature is not maintained during electrospinning, a portion of the nanofibers is dissolved in the solvent due to insufficient residual solvent volatilization, as illustrated FIG. 2.

<Experimental Example 2> Measurement of Performance of Unit Cell

In order to compare the performance of the fuel cells according to the present invention, the performance of the unit cells was measured under the following conditions.

Relative humidity: 80%

Cell temperature: 65° C.

Gas supply: anode—hydrogen/cathode—air

Measurement device: fuel cell performance TEST STATION by CNL Co.

Surface area of electrolyte membrane: 25 cm²

First, current-voltage values of the unit cells according to Examples 1-2 to 4-2 were measured, and thus current-voltage curves are illustrated in FIG. 3.

As illustrated in FIG. 3, it is confirmed that fuel cells of Examples 1-2 and 2-2 using gas diffusion layers produced by applying a voltage while maintaining the temperature of the polymer spinning solution in the supply container at 45° C. and 70° C., respectively during the production have superior power generation performance than those of Examples 3-2 and 4-2 using gas diffusion layers produced by applying a voltage while maintaining the temperature of the polymer spinning solution in the supply container at 35° C. and 25° C., respectively. Further, it is confirmed that the fuel cell of Examples 1-2 has further superior power generation performance than that of Examples 2-2.

In particular, as illustrated in FIG. 3, the current density ranging from 800 mA/cm² to 1200 mA/cm² corresponds to the mass transfer section in which the performance is decreased by the concentration gradient due to the discharge of water and inflow of gas in association with a pore size of the gas diffusion layer. It is confirmed that in this section, the performance degradation of the fuel cells of Examples 1-2 and 2-2 was lower than that of the fuel cells of Examples 3-2 and 4-2, and in particular, the performance degradation of the fuel cell of Example 1-2 was hardly caused in the mass transfer section. 

1. A gas diffusion layer comprising a carbon nanofiber spun layer for a fuel cell, wherein the carbon nanofiber spun layer is formed by electrospinning a polymer composition.
 2. The gas diffusion layer according to claim 1, wherein the electrospinning is performed by applying a voltage of 30 kV to 70 kV.
 3. The gas diffusion layer according to claim 1, wherein the electrospinning is performed at a temperature of 40° C. to 80° C.
 4. The gas diffusion layer according to claim 1, wherein the carbon nanofiber spun layer is formed by heat-treating the carbon nanofiber spun layer formed by the electrospinning.
 5. A membrane-electrode assembly for a fuel cell, the assembly comprising: an electrolyte membrane; and an anode electrode and a cathode electrode facing each other with the electrolyte membrane interposed therebetween, wherein each of the anode electrode and the cathode electrode includes a gas diffusion layer and a catalyst layer, and wherein the gas diffusion layer is the gas diffusion layer for a fuel cell
 6. A fuel cell comprising: a stack including one or more membrane-electrode assemblies and a separator interposed between the membrane-electrode assemblies; a fuel supply unit for supplying fuel to the stack; and an oxidant supply unit for supplying an oxidant to an electricity generating unit. 